1. Field of the Invention
The present invention relates to an electric power steering device for giving an assisting steering force to a steering mechanism of a vehicle by an electric motor.
2. Description of the Related Art
Conventionally, there is provided an electric power steering device in which an assisting steering force is given to a steering mechanism by driving an electric motor according to a steering torque given to a handle (steering wheel) by a driver. In this electric power steering device, there is provided a torque sensor by which a steering torque given to the steering wheel, which is a steering means, is detected and a torque detection signal indicating the steering torque is outputted. According to the torque detection signal sent from the torque sensor, a target value of the electric current, which is made to flow in the electric motor, is determined. According to the deviation between this target value and the detection value of the electric current actually flowing in the electric motor, a command value to be given to the drive means of the electric motor is generated. The drive means for driving the electric motor includes: a PWM signal generation circuit for generating a pulse width modulation signal (PWM signal) of a duty ration according to the command value; and a motor drive circuit composed of a power transistor turned on and off according to the duty ratio of the PWM signal. Voltage corresponding to the duty ratio, that is, voltage corresponding to the command value is impressed upon the electric motor. An electric current flowing in the electric motor by this impression of voltage is detected by the electric current detector, and a difference between this detection value and the above target value is used as deviation for generating the above command value. As described above, in the electric power steering device, feedback control is conducted so that the target electric current, which is set according to the steering torque indicated by the torque detection signal sent from the torque sensor, can flow in the electric motor.
The torque detection signal outputted from the torque sensor is a voltage signal corresponding to the steering torque. When the steering torque value is xe2x80x9c0xe2x80x9d, the voltage value of the signal concerned is referred to as a middle point voltage. This middle point voltage is shifted from a predetermined reference value due to a difference between the individual torque sensors and also due to deviation of the hardware into which the torque detection signal is inputted. Therefore, when each torque sensor is installed or the initial setting of the apparatus is conducted, either the middle point voltage value peculiar to the torque sensor concerned or the correction value for correcting a deviation from the above reference value is stored in the apparatus. This motion is referred to as xe2x80x9ca middle point adjusting motionxe2x80x9d, hereinafter, and a value stored here is referred to as xe2x80x9ca storage valuexe2x80x9d, hereinafter. By the constitution in which the middle point voltage is calculated according to this storage value, the above feedback control is conducted according to the middle point voltage which is not affected by the difference between the individual torque sensors.
As described above, in the electric power steering device, the torque value detected by the torque sensor for setting the target electric current value is an important factor. However, there is a possibility that the storage value obtained as a result of the above middle point adjusting motion becomes abnormal because of the abnormal motion of the storage device after that. In the case where the torque value is calculated according to this abnormal storage value, the electric current target value is set at an abnormal value. As a result, an abnormal assisting steering force is generated, and it causes that a driver""s good feeling of steering may be hurt and hazardous driving conditions (such as a slip-and-fall condition) may occur.
It is an object of the present invention to provide an electric power steering device not hurting a driver""s good feeling of steering even when the storage value is abnormal due to an abnormal motion of the storage device.
The first invention provides an electric power steering device for giving an assisting steering force to a steering mechanism of a vehicle by driving an electric motor according to a steering torque given to a steering means for steering the vehicle, comprising:
a torque sensor for outputting a voltage corresponding to the steering torque as a torque detection signal;
a middle point voltage storage means for storing a value corresponding to a middle point voltage which is a voltage outputted from the torque sensor when the steering torque is 0;
a steering torque calculation means for calculating the steering torque according to a torque detection signal, which is outputted from the torque sensor, and a value stored by the middle point voltage storage means;
an electric current detection means for detecting an electric current flowing in the electric motor and outputting a detection value of the electric current;
a motor control means for generating a command value used for feedback control conducted for driving the electric motor according to deviation of the electric current detection value from an electric current target value, which is set as a target value of an electric current to be supplied to the electric motor, according to the steering torque calculated by the steering torque calculation means;
a drive means for driving the electric motor according to the command value; and
a suppressing means for suppressing the assisting steering force in the case where the value stored in the middle point voltage storage means is out of a predetermined allowable range.
According to the first invention described above, in the case where the value stored by the middle point voltage storage means is out of a predetermined allowable range, the abnormal assisting steering force is suppressed by the suppressing means. Due to the above constitution, even when a value corresponding to the middle point voltage stored by the middle point voltage storage means is abnormal for the reasons of malfunction or others, an abnormal assisting steering force is suppressed as a result. Therefore, it is possible to prevent a driver""s good feeling of steering from being hurt and hazardous driving conditions.
The second invention provides an electric power steering device according to the first invention, wherein when the value stored in the middle point voltage storage means is not out of the allowable range, the suppressing means gives a value stored in the middle point voltage storage means to the steering torque calculation means, and when the value stored in the middle point voltage storage means is out of the allowable range, a predetermined value in the allowable range is given to the steering torque calculation means, and the steering torque calculation means calculates the steering torque using a value given from the suppressing means.
According to the second invention described above, in the case where a value stored by the middle point voltage storage means is out of a predetermined allowable range, a value in the predetermined allowable range is given to the steering torque calculation means by the suppressing means as a value corresponding to the middle point voltage. Due to this constitution, even when a value stored by the middle point voltage storage means is abnormal, there is no possibility that an abnormal assisting steering force is generated. Therefore, it is possible to prevent a driver""s good feeling from being hurt and hazardous driving conditions.
The third invention provides an electric power steering device according to the first invention, wherein when the value stored by the middle point voltage storage means is out of the allowable range, the suppressing means stops generating the assisting steering force.
According to the third invention described above, in the case where a value stored by the middle point voltage storage means is out of the predetermined value, generation of the assisting steering force is stopped. Due to this constitution, even when a value corresponding to the middle point voltage stored by the middle point voltage storage means greatly deviates exceeding the allowable range, no assisting steering force is generated as a result. Therefore, it is possible to positively prevent a driver""s good feeling of steering from being hurt and hazardous driving conditions.
The fourth invention provides an electric power steering device according to the first invention, the suppressing means including: an auxiliary storage means for storing the same value as the value stored by the middle point voltage storage means; and abnormality processing means for stopping generating the assisting steering force in the case where a difference between the value stored by the middle point voltage storage means and the value stored by the auxiliary storage means is not less than a predetermined value.
According to the fourth invention described above, in the case where a difference between the value stored by the auxiliary storage means and the value stored by the middle point voltage storage means is larger than a predetermined value, generation of the assisting steering force is stopped. Due to this constitution, even when a value corresponding to the middle voltage stored by the middle voltage storage means is abnormal, no assisting steering force is generated as a result. Therefore, it is possible to positively prevent a driver""s good feeling of steering from being hurt. In the case of constitution in which a sub-microcomputer, which is provided in ECU as an auxiliary storage means in many cases, is used, the structure of the apparatus can be made simple and the manufacturing cost can be reduced.